Knife-wielding traffickers sealed migrants in the hull of a boat which sank off Libya, condemning them to drown after slashing or thrashing them depending on their ethnicity, according to survivor testimonies.

Italy has arrested five men accused of multiple murders and human trafficking following last Wednesday's shipwreck, which left more than 200 people feared drowned.

Two Libyans, two Algerians and a Tunisian were held by police after survivors of the disaster were brought to Sicily, as testimony emerged as to how the suspects had beaten and stabbed passengers during the perilous crossing.

The alleged traffickers, aged between 21 and 24, were accused by witnesses of "branding with their knives the heads of those who did not obey orders, particularly Africans, while those of Arab origin were thrashed with belts", police said in a statement.

The overcrowded vessel was believed to have had more than 600 migrants onboard when it began the journey across the Mediterranean before getting into trouble and overturning.
Rescuers pulled 373 men, women and children from the water and recovered 26 bodies.

A search for further survivors proved fruitless.

Police said "the criminals each took on a clear role", with one in charge and the others tasked with controlling the migrants through violence.

Survivors told them people from Africa had been put in the hold, and that "they could be closed in and compacted in the hull for three days, having paid half price for the crossing".

Passengers said they had paid up to $US1,800 ($2,400) for the trip from the coast of north Africa to Europe, with those who wanted a life-jacket shelling out an extra 35 to 70 Libyan dinar ($34 to $68).

When water began seeping in, "the migrants, on the traffickers' orders, tried desperately to get rid of it", the police said.

When that failed "they did everything to try and get out to save themselves, but instead were attacked with knifes and sticks, pushed back into the hull", at which point the traffickers "sealed the hatch, with the weight of the rest of the migrants, positioned on purpose to stop it reopening".